Heavy duty liquid laundry detergents have achieved increasing consumer acceptance in recent years as convenient alternatives to conventional powder laundry detergent compositions. Liquid detergent products generally provide the consumer with a more concentrated and compact commercial detergent product relative to powder products and also allow for ease of operation with respect to targeting the removal of specific stains on fabrics, such as by a pre-spotting or pre-soak step prior to laundering the soiled fabrics in a home washing machine.
In order to achieve efficient laundering of fabrics, laundry detergent compositions are generally formulated to contain a variety of active ingredients, typically one or more anionic surfactants, often in combination with a nonionic surfactant, detergent builder materials such as alkali metal carbonates and zeolites, electrolytes and adjuvants such as brighteners, perfumes and colorants, all of which in the case of a liquid composition must be dispersed or dissolved in an aqueous medium. Because of the relatively limited solubility of this combination of ingredients in water, and the resultant tendency to form a thick, difficult to pour liquid often characterized by phase separation, the principal problems to be overcome in formulating a commercially desirable liquid detergent product are primarily stability and pourability, particularly for liquid products containing high levels of surfactants and builders.
Structured liquid detergent products have received much attention in the laundry detergent art as a means of providing liquid detergent compositions which are capable of suspending undissolved particles while remaining pourable and otherwise having acceptable rheological properties such as phase stability upon storage at ambient and at elevated temperatures. These structured liquids typically contain between 10% and 20%, by weight, of surfactants and from 10% to 25%, by weight, of detergent builder. The challenge in formulating this type of structured liquid is to prevent separation of the liquid into two or more layers while maintaining the viscosity at a pourable level which is commercially acceptable based on the preferences and requirements of the consumer.
The dispersed structuring phase in a structured liquid composition is believed to consist of an onion-like configuration wherein water or the aqueous phase is trapped between concentric layers of detergent active materials. These configurations are often referred to as lamellar droplets or micelles, and it is believed that it is the presence of such lamellar droplets dispersed in the aqueous phase which renders the liquid capable of suspending particulate solid materials, notably insoluble detergent builders, in the liquid product. The invention which is herein described refers to structured liquid detergent compositions which are exemplified by configurations of this type.
Cationic surfactants are extensively described in the patent literature as useful components of heavy duty liquid detergent compositions. But in practice, most commercial non-softening laundry products intended for home-use do not contain this type of surfactant, and instead, are predominantly comprised of a conventional sulfated or sulfonated anionic surfactant and/or a conventional ethoxylated nonionic surfactant primarily because of economic considerations. Moreover, there exists a general notion that anionic and cationic surfactants can not be used in the same composition without suffering a loss of efficacy due to the interaction of the positively charged cationic with the negatively charged anionic in the wash bath. Consequently, anionic and nonionic surfactants remain the "workhorse" surfactants in the majority of commercial liquid laundry detergent cleaning products.
Mixtures of anionic and cationic surfactants have been described in the literature as being useful for laundering compositions. U.S. Pat. No. 5,441,541 to Mehreteab et al describes certain complexes of anionic and cationic surfactants which are found to provide improved removal of oily stains from fabrics. More recently, a series of published PCT applications filed in the name of Procter & Gamble Company have focused attention on detergent compositions containing a bis-alkoxylated quaternary ammonium surfactant in combination with an anionic surfactant and a third component selected from among a soil dispersant polymer; a bleach material; a percarbonate bleach; and an enzyme among other additives useful for cleaning. Included among these published applications are the following: WO 97/44418; WO 97/44419; WO 97/44425; WO 97/44431; WO 97/44432; WO 97/43364; WO 97/43365 and WO 97/43371. The compositions described in these PCT publications do not refer to or otherwise contemplate being able to formulate such compositions in the form of a structured liquid.
Accordingly, there remains a need in the prior art for structured liquid laundry compositions capable of incorporating high levels of a mixed surfactant system of anionic and cationic surfactants in the presence of high levels of detergent builders and which have the capacity to efficiently clean oily soils and stains from fabrics. Further, there remains the need to provide structured liquid compositions which manifest a high degree of phase stability, and the requisite rheological properties to satisfy the commercial requirements for an effective liquid laundry detergent product.